The unit of productivity in an ecosystem is g m⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹. Productivity is a RATE of biomass production, so it must carry a time term (per year). Memory hook: "Productivity has a clock." If you see g m⁻² or kcal m⁻² WITHOUT "per year", that is biomass or energy, not productivity.
Adding the time term "per year" (yr⁻¹) turns a biomass or energy unit into a productivity unit. NEET tests this exact difference.
Your doubts, answered
What is the unit of productivity in an ecosystem?
The unit of productivity is g m⁻² yr⁻¹ (grams per square metre per year) or (kcal m⁻²) yr⁻¹ (kilocalories per square metre per year). NCERT gives both. The area term (m⁻²) lets you compare different ecosystems fairly, and the time term (yr⁻¹) shows it is a rate.
Why does the unit of productivity have 'per year' (yr⁻¹) in it?
Because productivity means the RATE of biomass production, not the total amount. A rate is always an amount divided by time. So the unit must have a time part. Without the yr⁻¹, the value is just biomass (weight) or energy at one moment, not productivity.
Is g m⁻² the unit of productivity or of biomass?
g m⁻² (or kcal m⁻²) is the unit of biomass or organic matter produced, measured at one point in time. It has NO time term. To make it productivity, you add per year: g m⁻² yr⁻¹. This one difference (the yr⁻¹) is the most common NEET trap.
Are the units the same for primary and secondary productivity?
Yes. Primary productivity (by producers) and secondary productivity (by consumers) both use the same unit, g m⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹, because both are rates of new organic matter formation per unit area per unit time. Only the source differs (plants vs consumers).
Why is the unit expressed per unit area (m⁻²)?
The m⁻² lets you compare productivity across ecosystems of different sizes. A big forest naturally produces more total matter than a small pond, but per square metre you can compare them fairly. NCERT says the per-area form is used 'to compare the productivity of different ecosystems'.
⚠️ The NEET trap ✗ kcal m⁻² or g m⁻² (choosing a unit with no time term) ✓ (kcal m⁻²) yr⁻¹ or g m⁻² yr⁻¹ — productivity is a RATE, so it must have per year 🧠 NTA loves to mix biomass units (no time) with productivity units (with time). Ask: does it have yr⁻¹? If NO, it is biomass/energy, not productivity.
Real NEET questions
2025
Which of the following is the unit of productivity of an Ecosystem?
A · kcal m⁻³
B · (kcal m⁻²) yr⁻¹ ✓
C · g m⁻²
D · kcal m⁻²
Solution: Productivity is the RATE of biomass production, so its unit must carry a time dimension. NCERT states it is expressed as g m⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹ to compare ecosystems. Options (a), (c) and (d) have no 'per year' term, so they express biomass or energy at one moment, not productivity. Hence (B).
2020
In relation to gross primary productivity and net primary productivity of an ecosystem, which one of the following statements is correct?
A · GPP and NPP are one and the same.
B · There is no relationship between GPP and NPP.
C · GPP is always less than NPP.
D · GPP is always more than NPP. ✓
Solution: NCERT: NPP = GPP − respiration losses (R). Since plants always use some GPP in respiration (R is positive), GPP is always greater than NPP. Both share the same unit (g m⁻² yr⁻¹), but GPP > NPP in value. So (D).
Solved Ecosystems NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
NCERT states productivity is 'expressed in terms of g m⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹ to compare the productivity of different ecosystems'.
What is the unit of biomass, and how is it different from productivity?
Biomass (organic matter) is expressed as weight g m⁻² or energy kcal m⁻² — with no time term. Productivity adds 'per year' (yr⁻¹) because it is a rate. The presence or absence of yr⁻¹ is the key difference.
Can productivity be written with per day instead of per year?
In principle any time unit works for a rate, but NCERT and NEET use per year (yr⁻¹). For NEET, always pick the option with yr⁻¹, specifically g m⁻² yr⁻¹ or (kcal m⁻²) yr⁻¹.
Why do both weight and energy units exist for productivity?
Because biomass can be measured either as mass (grams) or as its energy content (kilocalories). NCERT gives both g m⁻² yr⁻¹ (weight) and (kcal m⁻²) yr⁻¹ (energy) as valid units.
